1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging, and more particularly, to an improvement on a pulse sequence control for controlling generation, adjustment, and execution of a pulse sequence used in the nuclear magnetic resonance imaging.
2. Description of the Background Art
The nuclear magnetic resonance imaging is a technique for imaging microscopic chemical or physical information of an imaging target object by utilizing the so called nuclear magnetic resonance phenomenon according to which the nucleus having a characteristic magnetic moment placed in a homogeneous static magnetic field can resonantly absorb the energy of the high frequency magnetic field of a specific resonant frequency.
This nuclear magnetic resonance imaging is quite time consuming compared with other medical diagnostic imaging techniques such as an ultrasonic wave diagnostic imaging and an X-ray CT (Computed Tomography). As a consequence, the nuclear magnetic resonance imaging has been disadvantageous in that the image artifact can be caused easily by the respiration of the patient, and that it is rather difficult to image a moving target object such as a heart and a blood vessel. In addition, because of the lengthy imaging time required, it can be rather painful to the patient.
Now, the image artifact in the nuclear magnetic resonance imaging can be attributed partly to a factor related to the incompleteness of the radio frequency (RF) magnetic field and the gradient magnetic field, and partly to a factor related to the motion on the patient side. Of these two factors, the factor related to the motion on the patient side such as the body motion of the patient or the blood flow within the patient 's body can be reduced to some extent by devising the appropriate pulse sequence.
On the other hand, the incompleteness of the RF magnetic field is due to the non-linearity of an RF amplifier and an inhomogeneity of the RF, magnetic field, while the incompleteness of the gradient magnetic field is due to electric capacity factors of the gradient field amplifier, field distribution factors and inductance factors of the gradient field coils, as well as a transient factor such as an eddy current factor.
In order to compensate these incompleteness factors and minimize the image artifact, it is necessary to carry out the pulse sequence adjustment. However, because the contributions of various factors vary depending on the type of pulse sequence to be used, the pulse sequence adjustment must be made every time an imaging apparatus component such as the probe coil and the gradient field coils is changed. Consequently, the maintenance of the apparatus for the nuclear magnetic resonance imaging can be quite cumbersome as the number of different types of the pulse sequence to be used in the nuclear magnetic resonance imaging increases.
In addition, in the high speed (or ultra high speed) imaging method such as EPI (Echo Planar Imaging), FLASH (Fast Low Angle SHot), and Turbo FLASH which is capable of reducing the required imaging time, much larger image artifacts can be caused easily by a slight inhomogeneity of the static magnetic field or a slight deviation of the gradient field switching timing, compared with an ordinary imaging method such as the spin echo (SE) imaging. For this reason, it is preferable to carry out the adjustments of the magnetic fields and the pulse sequence for each patient, before taking the actual image. However, such adjustments require the execution of the pre-scanning solely for that purpose, and this in turn increases the required time for the image taking procedure as a whole.
Furthermore, such a high speed (or ultra high speed) imaging method has the problem in that, as the pulse sequence of such a high speed imaging method becomes faster and more complicated, it has become impossible to make a sufficient event control for the pulse sequence by the conventionally available sequence controller.
Namely, there are two conventionally known procedures for the event control by the sequence controller, as follows.
(1) The pulse sequence data for taking a single image including a wait period due to the repetition time TR are stored in the event memory, and then the stored pulse sequence on the event memory is executed. In this procedure, the host computer side does not at all participate in the pulse sequence execution. In other words, all the operations to be executed for obtaining a single image are stored in the event memory first, and then the pulse sequence is actually executed according to the event memory by the sequence controller, quite independently from the host computer.
(2) The pulse sequence data for only a limited number of RF excitations is stored in the event memory, and then the portions of the event memory related to the encoding steps and the multi-slices are rewritten during the wait period due to the repetition time TR. In other words, the basic procedure is predetermined, and the specific parameters of this basic procedure are sequentially rewritten during the wait period while the pulse sequence is executed sequentially.
However, the procedure (1) has the problem in that the amount of data for the pulse sequence that can be stored in the event memory simultaneously is actually limited in practice, and the event memory in the conventionally available sequence controller may not have a sufficient memory capacity to deal with the highly complicated pulse sequence, or else the length of the pulse sequence such as that specified by the number of encoding steps must heavily depend upon the memory capacity of the event memory.
In addition, the procedure (1) also has the problem in that the control is not handed over to the host computer side at all during the execution of the pulse sequence, so that the operation to transmit the acquired data to the host computer side cannot be carried out until the entire pulse sequence is finished, and consequently the time available for the image re-construction on the host computer side could be quite limited.
Despite of all these drawbacks, for the ultra high speed imaging method such as Turbo FLASH which has a very short repetition time TR, only this procedure (1) is applicable.
On the other hand, the procedure (2) is advantageous in that it can deal with any complicated pulse sequence by utilizing the rewriting of the event memory, and the transmission of the acquired data can be made during the event memory rewriting operation.
However, this procedure (2) has the problem in that, in order to minimize the repetition time TR, it becomes necessary to provide the hardware for effectively shortening the rewriting process of the event memory and the data structure for effectively minimizing the number of steps required in the rewriting operation, but such hardware and the data structure have been unknown. Also, there are some rather complicated pulse sequences for which the repetition time TR cannot be shortened very much.
Also, in the high speed imaging method such as EPI, it is desirable to execute the pulse sequence interactively, such that the imaging slice plane can be changed sequentially according to the commands entered from the host computer side by the operator for example, so as to take a full advantage of the instantaneous imaging achievable by such a high speed imaging method.
However, such a high speed imaging method requires a very large pulse sequence data set, so that in the conventional event memory rewriting procedure (2) involving the direct participation of the host computer side, the rewriting of the event codes cannot be completed within an ordinary repetition time TR (about 100 ms). In other words, it has been impossible conventionally to sufficiently speed up the operations for rewriting the event memory, such that it has been difficult to realize the interactive pulse sequence execution even when the high speed imaging method capable of obtaining the continuous real time images is employed.
In addition, in the conventional sequence controller, because of the low time resolution realizable, the adjustment of the pulse sequence has been achieved in terms of the amplitude level of the gradient magnetic field, but this procedure for the adjustment of the pulse sequence can be affected by the eddy currents more easily compared with a case of controlling the pulse sequence with the fixed amplitude on the time axis, so that it is not suitable for the high precision adjustment of the pulse sequence.